1. Field of the Invention
This invention relates to anabaseine, DMAB-anabaseine, and anabasine and their use to treat degenerative diseases of the nervous system.
2. Description of Related Art
It has long been customary in classifying diseases of the nervous system to group them as degenerative, thereby indicating they are characterized by a gradually evolving, relentlessly progressive, neuronal death. Science has shown that a considerable portion of disorders that are classed as degenerative are associated with genetic predisposition which results in a pattern of dominant or recessive inheritance. However, others, although they do not differ in a fundamental way from the hereditary disorders, may occur only sporadically as isolated instances within a given family.
As a consequence, since by definition, classification of degenerative diseases cannot be based upon exact knowledge of their cause or pathogenesis, subdivision of these diseases into individual syndromes rests upon descriptive criteria based largely upon pathologic anatomy and consideration of clinical aspects. As a result, this group of diseases presents itself in the form of several clinical syndromes. However, apart from the general differences that allows the distinction of one syndrome from another, there are certain general attributes which typify this entire class of disorders.
The degenerative diseases of the nervous system can typically be divided into disorders characterized by progressive dementia in the absence of other prominent neurologic signs (e.g., Alzheimer's disease, senile dementia, and Pick's disease); syndromes which combine progressive dementia with other prominent neurologic abnormalities (e.g., Huntington's disease, Hallervorden-Spatz, and progressive familial myoclonic epilepsy); syndromes of gradually developing abnormalities of posture and movement (e.g., Parkinson's disease, striatonigral degeneration, torsion dystonia, and Gilles de la Tourette syndrome); syndromes of progressive ataxia (e.g., cerebellar cortical degeneration, olivopontocerebellar atrophy, and Friedreich's ataxia); and syndromes of muscular weakness and wasting without motor neuron disease (e.g., amyotrophic lateral sclerosis, spinal muscular atrophy, and hereditary spastic paraplegia), to name but a few.
Among those diseases listed above, perhaps those most familiar are Alzheimer's and Parkinson's diseases. These diseases are progressive neurological disorders characteristically associated with aging. Alzheimer's disease is characterized by a profound loss of memory and other cognitive functions, while Parkinson's disease is an extrapyramidal movement disorder. Both are invariably fatal. Although there is no effective treatment for Alzheimer's disease, clinical trials are underway with several drugs that increase brain cholinergic transmission. In Parkinson's disease, several treatments are temporarily useful, notably L-DOPA related therapies that replace dopamine in the nigrostriatal pathway. However, in Parkinson's disease the therapeutic efficacy of even the best drugs is temporary at best.
Although the loss of neurons in the late stages of Alzheimer's disease is profound, only a few neuronal pathways appear to be affected in its earliest stages. These include cholinergic projections from the nucleus basalis to the cerebral cortex and from the septum to the hippocampus, noradrenergic projections from the locus cerululus to the cerebral cortex, and several peptidergic neurons that are probably intrinsic to the cerebral cortex. The loss of the aforementioned cholinergic pathways in particular is believed to underlie the early memory loss, since these pathways are known to be important for memory and cognition. This association accounts for the major emphasis in novel cholinergic treatments for Alzheimer's disease, at least in its early stages.
A recent study on Alzheimer's disease demonstrated that loss of cholinergic projections from the nucleus basalis to the cerebral cortex was sufficient, after extended intervals, to cause trans-synaptic neuron loss in the rat. Thus, it is conceivable that the early loss of analogous cholinergic neurons in Alzheimer's disease could cause a profound cascade phenomenon resulting in the loss of many neurons over a period of years. If so, then replacement therapy might not only improve survival of these neurons, but perhaps more important, keep other brain cells from dying.
Given the possibility of such therapy, it is of primary importance to determine the type of cholinergic agent most likely to improve memory and/or keep brain neurons from dying after the loss of cholinergic neurons. To address this issue, it is necessary to consider the two general types of cholinergic transmission in the brain. One is termed muscarinic, the other nicotinic. These terms are based on the type of receptor to which acetylcholine binds to in order to elicit its neurotransmitter effect. In brain regions associated with memory, the muscarinic receptors predominate quantitatively over the nicotinic receptors, although both types coexist. For this reason, most investigators traditionally focused on the development of muscarinic agonists to improve memory-related behaviors. These agents have been found to have moderate effects in rats with lesions of the nucleus basalis, but have little effect in patients with pronounced Alzheimer's disease.
There is reason to believe, however, that nicotinic transmission may also be important for treating Alzheimer's disease. This is supported by the fact that cerebral cortical nicotinic receptors decrease significantly during the disease, while post-synaptic muscarinic receptor levels are often unchanged. These observations are consistent with the hypothesis that neurons expressing nicotinic receptors are lost in the disease. When these observations are combined with those of the present inventors, that lesions of ascending cholinergic neurons from the nucleus basalis cause a trans-synaptic neuron loss in the cortex, it is hypothesized that the neurons in the cortex that die trans-synaptically (and in Alzheimer's disease) do so because they do not receive enough nicotinic stimulation. For this reason, the inventors believe nicotinic agents may be useful as replacement therapy for keeping brain neurons alive in Alzheimer's disease that would otherwise die from lack of nicotinic transmission. An analogous situation exists in several other systems such as: (a) muscle cells, which atrophy in the absence of nicotinic activation; (b) sympathetic ganglia, which require either nerve growth factor or nicotinic transmission (in the presence of calcium ions) in order to survive in culture; and (c) nigrostriatal dopamine neurons, which appear to be partially spared by nicotine following lesions of the substantia nigra. Also, it is important to note that there exist several types of nicotinic receptors in the brain, which allows considerable potential selectivity in targeting drugs for certain nicotinic sites.
The observation that nicotine treatment can preserve nigrostriatal dopamine neurons in an animal model for Parkinson's disease is consistent with epidemiological evidence that there is a lower incidence of this disease in cigarette smokers (even after adjusting for the smoking-induced increase in mortality). The mechanism whereby nicotine can preserve these neurons is not known, but it does appear to involve effects of nicotinic transmission on dopamine neurons themselves, since these neurons possess this type of cholinergic receptor. While the remainder of this patent application focuses on the potential treatment of Alzheimer's disease with nicotinic receptor agents, it should be noted that these drugs may be just as effective, or more so, on dopaminergic neurons that are lost in Parkinson's disease.
Nicotine has been used in several clinical trials for the treatment of Alzheimer's disease, primarily over rather short intervals for its potential memory enhancing effect (not for its ability to block long term trans-synaptic cell loss). In one recent study, nicotine had a marginally positive effect on memory and an even greater one of improving the mood of the patients. These positive results have not been followed up with longer term ones, however. Unfortunately, while nicotine has a history of improving memory related behaviors in humans and animals, its potent toxicity, low effective dose range, and peripheral side effects, have basically rendered it unacceptable for treating Alzheimer's disease.
Thus, considerable need exists for agents which stimulate cholinergic transmission, but, unlike nicotine, are relatively non-toxic. The present invention provides a method of using agents which have this capability.